Oscillograph apparatus



March 3, 1959 H. 1. CHAMBERS ETAL 2,876,059

OSCILLOGRAPHAPPARATUS Filed June 18, 1956 5 Sheets-Sheet 1 FIG spa-5o CONTROL 3/ 97 W TO ELECTRODE lNVE/V TORS.

LE WIS B. BROWDER HERBERT CHAMBTRS A TTORNEYS March 1959 H. I. CHAMBERS ETAL 2,876,059

OSCILLOGRAPH APPARATUS Filed June 18, 1956 3 Sheets-Sheet 2 M/VE/VT'QRS. LEWIS B. BROWDER HERBERT CHAMBERS PMAeM A TTORNEYS Marc]? 1959 H. CHAMBERS ET AL 2,876,059

OSCILLOGRAPH APPARATUS 3 Sheets-Sheet 3 Filed June 18, 1956 TO SPEED CONTROL 7'0 ELECTRODE WR/ TING POWER INVENTOPS. LEW/S B. BROWDER OUTPUT VOL T4 GE HERBERT l CHAMBERS SIGNAL n came/w g 7 a J ATTORNEYS United States Patent OSCILLOGRAPH APPARATUS Herbert I. Chambers, Pasadena, and Lewis B. Browder, La Canada, Califi, assignors to Consolidated Electrodynamics Corporation, Pasadena, Calit., a corporation of' California Application June 18, 1956, Serial No. 592,209

13 Claims. 01. 346-74 This invention relates to improvements in string galvanometers which record oscillographic traces directly upon a recording medium such as current-sensitive paper.

In a typical string galvanometer, the string or writing element is an elongated movable conductor. The conductive string is located in a magnetic field perpendicular to the direction of the magnetic field. The string is supported at its two ends so that its central portion can be deflected transversely with respect to the magnetic field a relatively large distance to provide large amplitudes of motion which are required for recording directly.

Preferably, the string is supported at each end by springs so that as the string is deflected in either direction, the springs permit the ends of the string to move closer together.

For direct writing on current-sensitive recording paper, an anvil having an edge located at the central portion of the string is employed to guide the recording paper adjacent the center of the string. The edge of the anvil is perpendicular to the string, and the current-sensitive paper is moved over the edge of the anvil so that itpasses between the anvil and the string. Suitable means are provided for urging the string against the paper where the paper passes over the edge of the anvil, and a source of writing current is coupled between the paper and the string so that an electric current passes between the paper and 'the string at the intersection of the string and the line formed where the recording paper passes over the edge of the anvil. The current density at the point of contact between the paper and the string is sufficiently high to turn the paper black at that point. Thus, a trace is formed on the paper which provides a record of the deflections of the string in response to an applied signal.

This type of direct recording presents the problem of maintaining a constant current density where the string contacts the paper to prevent burning of the paper due to current density which is too high, or to prevent the fading of the trace due to a current density which is too low. This problem is aggravated by the fact that the paper is usually supplied current by sliding over an electrode, and the contact resistance between the electrode and the paper varies with the speed at which the paper slides past the electrode, and may even be erratic at a constant paper speed.

Further difliculties are encountered when the string is recording an oscillating signal which causes the speed of the string relative to the paper to vary with the signal amplitude. For example, if a sine Wave is recorded, the velocity of the string is maximum at zero deflection and is zero at maximum deflection. Under these conditions, the contact resistance between the string and the paper is a maximum at zero deflection and a minimum at maximum deflection of the string, which tends to produce a trace which is faint at zero deflection and which is dark at maximum deflection.

This invention provides a direct recording oscillograph which produces a trace of uniform density on currentsensitive paper in spite of varying contact resistance between the paper and the charging electrode, and between the paper and the string.

Briefly, the invention contemplates a circuit which includes means for applying a voltage between the currentsensitive paper and the string, and means responsive to the amount of current flowing through the circuit to vary the voltage applied between the paper and the string to maintain a substantially constant current density at the contact between the paper and the conductor. Preferably, the circuit includes a cathode-biased vacuum tube operating as a dynamic series resistor to maintain a substantially constant current between the paper and the string.

In another form of the invention, the current is maintained substantially constant by the use of a large series resistor in the circuit, so that fluctuations in the contact resistance of the electrode or string with the paper are negligible compared to the large series resistor.

Another embodiment to compensate for variations in contact resistance between the paper and the string due to an oscillating signal includes a phase shifting network to use the signal to modulate the voltage applied between the string and the electrode. With this circuit, the voltage is increased to a maximum when the signal cur-' rent is zero (corresponding to zero amplitude for the string) and is reduced to a minimum value when the signal current is at a maximum. 7

The invention will be more fully understood from the following detailed description and accompanying drawings in which:

Fig. 1 is a fragmentary, partially schematic, elevation. of a string galvanometer to which this invention is applicable;

Fig. 2 is a sectional view along line 2-2 of Fig. 1;

Fig. 3 is an enlarged detail plan view of a means for anchoring the ends of the string;

Fig. 4 is an elevation, partly broken away, of the anchoring means of Fig. 3

Fig. 5 is a schematic diagram of a circuit using a vacuum tube as a dynamic series resistor for maintaining a substantailly constant writing current;

Fig. 6 is a schematic diagram of a circuit using a large resistor in series with the string and electrode so that fluctuations in contact resistance between the paper and the electrode or string are negligible;

Fig. 7 is a schematic diagram of a circuit using a phase shifting network which modulates the writing voltage in accordance with the signal to compensate for fluctuations in contact resistance between the string and the paper; and

Fig. 8 is a graph showing the relationship between the signal and the modulated writing voltage produced by the circuit of Fig. 7. e

For simplicity, the following detailed description is limited to a single channel oscillograph, although the invention is applicable to oscillographs using any number of channels.

The string shown in Figs. 1 and 2 is an elongated flexible conductor 10 supported at opposite ends by a pair of leaf springs 12. A bearing sleeve 14 of ferromagnetic material is coaxially disposed around and rigidly attached to the center of the string. The leaf springs are mounted at opposite ends of a base strip 16 which is attached by bolts 18 across the upper portion of an upright, U-shaped magnet yoke 19.

The magnet yoke supports an upper pole piece 20 formed of right and left (as viewed in Fig. 1) upper pole members 21, 22 respectively, each resting on an upper end of the yoke. The upper'pole members are colinear and spaced apart. A writing anvil 24 is mounted between the upper pole members and has an elongated washers and sleeves.

bottom edge 25 contoured to permit the smooth passage of a strip of current-sensitive recording paper 26 to pass over it between the pole pieces and in direct contact with the bearing sleeve on the string. The paper moves from a supply reel 28 to a take-up reel 29 over guide rollers 30. The reels are powered by suitable means (not shown), and their operation is controlled from a speed control box 31, which has a plurality of control buttons 32 which start, stop, and change the speed of the reels. Control of the reels may be achieved either electrically, or through a suitable variable speed transmission, or by any other suitable arrangement. The speed control of the reels control does not form any part of this invention, and therefore a detailed description of it is omitted. The anvil edge is perpendicular to the string and extends for a distance suflicient to support the paper for as many channels as may be used.

A lower pole piece 33 is located directly below and spaced from the upper pole members. The string is disposed in the space between the upper and lower pole members and is perpendicular to the flux between the upper and lower pole pieces, which are connected by the yoke. The sleeve is magnetically attracted toward the anvil and maintains the proper writing pressure against the paper.

A separate mounting bar 34 is attached by screws 35 near each end of the base strip and extends perpendicularly to the strip into the respective space between the magnet yoke and each side of the lower pole piece. Each bar is further secured to the yoke by a key 36 slip-fitted into a groove 37 in each side of the bar nearer the yoke. Additionally, each key is press-fitted into a separate groove 38 in the yoke and extends parallel to its respective mounting bar for almost the entire length of the bar. A separate, externally threaded mounting pin 39 for the flexible conductor extends vertically (as viewed in Fig. 1) through a separate bore 40 (see Fig. 4) in each mounting bar, and is secured to its respective mounting bar by a separate nut 41 threaded onto the lower end of each mounting pin. A separate insulating washer 42, which may be of extruded fiber, is disposed around the upper end of each pin and extends down into its respective bore 40 to space and insulate each pin from the bar in which it is mounted. The pin in the right hand (as viewed in Fig. 2) bar is near the end of the bar remote from the base strip, and the pin in the left hand bar is near the end of the bar adjacent the strip. A separate insulating sleeve 43, which also may be of extruded fiber, is disposed around the lower end of each pin and held up against the under side of the respective bar by the respective nut on each pin. Thus each pin is electrically insulated from its bar by theinsulating Electrical connection is made to the pins and each end of the flexible conductor by leads 44 soldered to the lower end of the pins and connected to opposite sides of a signal source 45.

The detail construction of the pins is best explained by reference to Figs. 3 and 4 which show the left hand mounting pin in detail. The pins are identical, and the following description for the left hand pin will sufiice for the right hand pin. The pin has a cylindrical head 48 with a slot 49 extending across a chord of the head near its center. A first raised four-sided boss 50 on the right hand (as viewed in Figs. 3 and 4) side of the head lies between the slot 49, a narrow groove 52 extending parallel to the slot, and a wider groove 53 extending inwardly from the periphery of the head to the edge of the slot. The groove 53 is perpendicular to the slot. The grooves 52 and 53 are not as deep in the head as is slot 49, which receives a screw driver during the mounting of the pin in the bar. Grooves 52 and 53 have bottom surfaces which are in a plane common to a level portion 54 of the head, which lies on the side of the slot opposite from grooves 52 and 53.

A second raised four-sided boss 56 is on the left hand side of the head between slot 49 and narrow groove 52. The left hand boss has a tapped horizontal bore 58 in its outermost portion to receive a set screw 59. The leaf spring 12 supporting the left end of the string 10 is mounted in the pin shown in Fig. 4. An end 60 of the leaf spring is bent at right angles to the intermediate portion of the spring. The spring is bent at the opposite end 61 in a gradual curve to form a free end extending in the same direction as the end 60. Free end 61 is diminished in both width and thickness toward its tip to provide a free end with a small mass and the ability to vibrate in the frequency range required by the flexible conductor. End 60 of the spring rests in groove 52 and is held in place in the groove by a wedge 62, which is kept in the goove by peening the edge of the groove as shown at 63. The intermediate portion of the leaf spring extends along the groove 53 and is held against boss in a cantilever position over the flat portion 54 of the head by an L-shaped clamp 64 and set screw 59. One leg of the clamp lies in groove 52 where it is slidably retained by peening the edge of the groove at 65, and the other leg lies in Wider groove 53. i

The string has a flattened portion 66 adjacent each of its ends. Flattened portion 66 at the left end of the conductor, as viewed in Fig. 4, extends along the convex surface of the leaf spring so that the major dimension of the flattened portion is vertical. The flattened portion is held between the L-shaped clamp and the intermediate portion of the leaf spring. A cylindrical tip 67 is left on the end of the conductor and acts as an index to position the conductor by contacting the curved portions of the L-shaped clamp and the leaf spring. The flattened portion of the spring is cemented at 68 to the spring to restrain the string from vibration in any direction other than in the plane perpendicular to the magnetic flux.

The string is mounted at the opposite end as described above so that the desired tension is imparted to the flexible conductor. For instance, in Fig. 3, cantilever spring 12 assumes the free position shown by dotted lines 69. The distortion of the spring under the full vibration amplitude of the conductor is indicated by dotted lines at 70.

The flattened portion of the conductor is in electrical contact with the mounting pin, and inmechanical contact with the leaf spring from the clamped spring end to near its free end 61. Since the thickness of the conductor is reduced in the direction of conductor deflection, the conductor bends easily in this direction, while resisting bending in the direction normal to deflection. The vibration of the conductor causes the conductor to bend in that portion nearest the free end of the leaf spring. Since the leaf spring gives due to the added tension caused by deflection of the conductor, the point of tangency of the conductor to the curved free end of the spring changes and shifts the conductor bending point 7 accordingly. Thus, the bending strain is distributed as the conductor vibrates and the conductor is less susceptL ble to fatigue.

The right hand leaf spring is similarly mounted in the right hand pin so that its free end is cantilevered away from the pin and curves toward the left hand spring, and the right end of the flexible conductor is mounted on the right leaf spring as just described for the left end of the conductor. spective mounting bars so that the leaf springs are on opposite sides of the flexible conductor as shown in Fig. ,2.

When the flexible conductor is not subjected to current flow, it assumes a position midway between the full line position of conductor 10 and the dotted line position 10A in Fig. 2. In this unexcited condition the bearing sleeve occupies a central position with respect to the lower pole piece. When current flows through the conductor, the bearing sleeve moves from side to side of a central line 72 as it vibrates from peak to-peakof its oscillation.

The pins are arranged on their re-- The displacement transverse to the direction of vibration is due to the mounting of the leaf springs on op posite sides of the conductor. This transverse motion distributes the writing wear over a substantial portion of the'length of the bearing sleeve and also keeps the writing surface of the bearing sleeve wiped clean.

The central portion'of the upper surface of the lower pole piece is cut out as best shown in Fig. 2 so that the magnetic held in which the bearing sleeve moves in creases as the amplitude of the conductor increases. This magnetic flux distribution helps to compensate for the non-linear effects introduced by the fact that the amplitude induced by current flow varies inversely with the tension on the flexible conductor. V

Referring to Figs. 1 and 5 an electrode 74 is mounted to makesliding contact with the side of the paper contacted .by the string. The electrode is supplied voltage from a writing current circuit 76, which is shown in Fig. 5.

The circuit shown in Fig. 5 includes a pentode vacu-. um tube -79having its plate connected to the electrode 74 by a-lead 81. The vacuum tube is supplied operating voltage from a twin-diode full wave rectifier vacuum tube 82 center tapped across the secondary coil 83 of a transformer 84. Theprimary coil 85-of transformer 84 is connected to a source (not shown) of 110 volt, 6O cycle current. A cathode resistor R is connected to the cathode of the pentode to supply a cathode bias to the control grid 87 of the pentode. The bias applied to the control grid is adjustable through the sliding-contact 88, which is ganged to the control buttons on the control box to switch to a less negative position as the paper speed is increased. The pentode screen grid 90 is biased through lead 91 having an adjustable contact 92 adapted to slide up and down a resistor R connected to the negative power lead. A resistor R connected in series with the negative power lead and a capacitor C across the power leads in combination have the proper time constant .to smooth the pulsating D. C. into a substantially constant D. C. supply for the pentode. Voltage to heat the filaments of the twin-diode is supplied from coil 93, and the cathode heater voltage for the pentode is supplied from coil 94'. The suppressor grid 95 of the pentode is connected to the cathode. A shunt resistor R is connccted in parallel with the electrode.

'The operation of the oscillograph using'the circuit of Fig.,5 is as follows:

The 'oscillograph is turned on so that the recording paper is pulled over the edge of the anvil, and a signal is applied to the string. The writing circuit is turned on so that operating voltage is applied to the pentode, and so that the plate output of the pentode is connected to the electrode which is in contact with the paper. The electrode contacts the paper over a sufficiently large area so that the current density between the electrode and the paper is below the marking level at any point. The string moves over the paper in accordance with the applied signal. The string is grounded and the current density at the point of contact between the string and the paper is sufficiently high to cause the paper to turn black or be marked. A trace is thus formed on the paper which provides a direct recordingof the deflections of the string in response to the applied signal.

Even though the speed of the paper is constant,'the contact resistance between the electrode and the paper is erratic. If the contact resistance between the electrode and the paper increases, tending to reduce the amount of writing current flowing between the string and the paper, the bias applied to the pentode control grid through the cathode resistor decreases, and causes the tube to become more conductive, thus increasing the voltage applied to the electrode to maintain a substantially constant current output, and consequently a uniform writing current density at a point of contact between the paper and the string. If the contact resistance suddenly decreases, the pentode resistance is increased to keep the current from 'r-ising to an abnormally high value which might burn .the'paper or'make the trace-so dark as to be useless. In a similar. manner, if the contact resistance between 'thest-ring andthe paper tends tochan'ge due to variations of the speed of the string on the paper, the bias on the control grid of the pentode changes automatically to adjust the conductivity of the pentode to maintaina substantially constant writing current.

To provide a wider range of compensation, the movable contact 88 is ganged to the, buttons on the-speed control box so that the contact is automatically moved to a less negative position as paper speed is increased, and moved tea more negative position as-the paper speed is decreased. a

The circuit shown in Fig. 5 may be constructed of numerous combinations of components; however a satisfactory circuit wasrnade usingcomponents having the characteristics shown in Table I.

Table I R 1K R K R 1K.v R 100K C 20 mfd. Twin diode Tube type 5Y3. Pentode Tube type 6BK5.

The circuit made with components having the characteristics shown in the above table Worked very well with Time-fax current-sensitive paper having a semi-conductive coating which required a potential difference of about volts or more, coupled with high current density, to'turn black.

Another circuit for compensating for changing'contact resistance is shown in Fig. 6. This circuit, although satisfactory, does not perform as well as'the cir cu'it of Fig. 5. In the circuit of Fig. 6, a large resistance; R is connected in series with the" output of adiode vacuum tube rectifier 96 which supplies writing current to the electrode through a lead 97. Operating voltage for the diode is supplied through a secondary coil 98 of a transformer 99. Heating current for the diode cathode is furnished through a secondary coil 100 of the transformer 99. Resistors R and R arein series with each other and are connected in parallel with series connected capacitors C and C providing smoothing for the pulsating D. C. furnished by the diode. In this circuit a high voltage, say 2000 volts, is applied 'to the resistor R When the contact resistance between the paper and the string or electrode increases, the voltage applied to the electrode increases due to the lower voltage drop across the resistor R thus compensating for the change in contact resistance.

A suitablecircuit such as that shown in Fig. 6 was made using components having the characteristics shown in Table II. I

Table II R "500K (adjustable). R 4 megohms. R 4 megohms. C .04 mfd. C .04 mfd.

The setting for the resistor R can easily be determined experimentally for various types of paper and operating conditions, but generally the resistance of resistor R should be at least three or four times the contact resistance between the paper and the electrode and the paper and the string.

The circuit of Fig. 7' is another means for compensating for changes in writing contact resistance.- In this circuit the signal voltage is applied to a phase shifting network 101 which includes an isolation transformer 102, a variableresistor R resistors R and R and a variable ca pacitor C all connected as shown across the secondary winding of the transformer. The input signal is applied across a variable resistor R which is connected through a movable tap 103 across the primary winding of the transformer. The output of the phase shifting network, which is shifted 90 out of phase with the input, is passed through a rectifier 104 and applied to the control grid 106 of a'pentode, through a lead 107 having a movable contact 108 adapted to slide up and down a resistor R connected across the rectifier output. The pentode is supplied an operating voltage, say 350 volts, from a suitable source 109, and the suppressor grid 110 of the pentode is connected to the cathode 111. The screen grid 112 is supplied voltage through a lead 113 and movable contact 114 adapted to slide on resistor R connected between the cathode and ground. A biasing battery 115 is connected between movable contact 108 and the control grid. The output of the plate of the pentode is con nected to the electrode and is also grounded through resistor R In Fig. 8, a typical signal input current to the string and phase shifting network is represented by a sine curve 116, and the output voltage of the pentode, as modulated by the signal current, is represented by a curve 117. The output voltage from the pentode is a minimum when the signal current is a maximum, and the output voltage is a maximum when the signal is a minimum. Thus when the string velocity is at a maximum (zero signal current) the voltage applied to the charging electrode is also at a maximum. Conversely, when the string velocity is zero (maximum signal current) the voltage applied to the charging electrode is at a minimum. Therefore, with this circuit the voltage applied to the charging electrode is increased as the wn'ting contact resistance between the paper and the moving string increases, and is reduced as that contact resistance decreases.

As with the other circuits, numerous combinations of individual components can be used satisfactorily in the circuit of Fig. 7. The following Table III gives the characteristics of the various components which provide a suitable circuit.

TABLE III R 100K (adjustable). R 20K. R 20K. R 1K (adjustable). R 100K. R 75K. R 100K. C 1.1 mfd. (adjustable). Pentode Tube type 6BK5.

We claim:

1. An oscillograph comprising means forproducing a magnetic field, an elongated movable conductor, means for supporting the conductor in the magnetic field with the conductor extending transverse to the direction of the magnetic field, means for supporting and moving current-sensitive paper past the conductor in the direction of the longitudinal axis of the conductor, means for urging the conductor toward the paper so that contact is made between the conductor and the paper, a circuit for imposing a voltage between the conductor and the paper to cause a current to flow between the paper and the conductor with sufiicient density to mark the paper, and means for continuously changing the voltage to maintain substantially constant the current at the contact between the paper and the conductor.

2. An oscillograph comprising means for producing a magnetic field, an elongated movable conductor, means for supporting the conductor in the magnetic field with the conductor extending transverse to the'direction of the magnetic field, means for supporting and moving current-sensitive paper past the conductor in the direction of the longitudinal axis of the conductor, means for urging the conductor toward the paper so that contact is made between the conductor and the paper, a circuit for imposing a voltage between the conductor and the paper to cause a current to flow between the paper and the conductor with sufficient density to mark the paper, means for sensing the amount of current flowing between the paper and the conductor, and means responsive to the amount of sensed current to vary the voltage applied between the paper and the conductor to maintain substantially constant the current at the contact between the paper and the conductor.

3. An oscillograph comprising means for producing a magnetic field, an elongated movable conductor, means for supporting the conductor in the magnetic field with the conductor extending transverse to the direction of the magnetic field, means for supporting and moving current-sensive paper past the conductor in the direction of the longitudinal axis of the conductor, means for urg ing the conductor toward the paper so that contact is made between the conductor and the paper, a circuit for imposing a voltage between the conductor and the paper to cause a current to flow between the paper and the conductor with sufficient density to mark the paper, and an electron tube connected in the circuit as a dynamic series resistor for maintaining substantially constant the current at the contact between the paper and the conductor.

4. An oscillograph comprising means for producing a magnetic field, an elongated movable conductor, means for supporting the conductor in the magnetic field with the conductor extending transverse to the direction of the magnetic field, means for supporting and moving current-sensitive paper past the conductor in the direction of the longitudinal axis of the conductor, means for urging the conductor toward the paper so that contact is made between the conductor and the paper, a circuit for imposing a voltage between the conductor and the paper to cause a current to flow between the paper and the conductor withsufficient density to mark the paper, and a pentode vacuum tube connected in the circuit as a dynamic series resistor for maintaining substantially constant the current at the contact between the paper and the conductor.

5. An oscillograph comprising means for producing a magnetic field, an elongated movable conductor, means for supporting the conductor in the magnetic field with the conductor extending transverse to the direction of the magnetic field, means for supporting and moving current-sensitive paper past the conductor in the direction of the longitudinal axis of the conductor, means for urging the conductor toward the paper so that contact is made beween the conductor and the paper, a circuit for imposing a voltage between the conductor and the paper to cause a current to flow between the paper and the conductor with sufiicient density to mark the paper, and an electron tube with a cathode-biased control grid connected in the circuit for maintaining substantially constant the current at the contact between the paper and the conductor.

6. Apparatus according to claim 5 which includes means for changing the paper speed, and means for changing the bias on the control grid of the electron tube.

7. Apparatus according to claim 6 which includes means for automatically making the control grid less negative when the paper speed is increased and for antomatically making the grid more negative when the paper speed is decreased.

8. An oscillograph comprising means for producing a magnetic field, an elongated movable conductor, means for supporting the conductor in the magnetic field, with the conductor extending transverse to the direction of the magnetic field, means for applying to the conductor a signal, means for supporting and moving current-sensi tive paper past the conductor in the direction of the lonshifting network having an input and an output, means for applying the signal to the network input, and means for applying the network output to the writing circuit to increase to a maximum the voltage applied between the paper and the conductor when the signal is one value and to decrease to a minimum the voltage between the paper and the conductor when the signal is a higher value.

9. An oscillograph comprising means for producing a magnetic field, an elongated movable conductor, means for supporting the conductor in the magnetic field with the conductor extending transverse to the direction of the magnetic field, means for applying a signal to the conductor, means for supporting and moving current-sensitive paper past the conductor in the direction of the longitudinal axis of the conductor, means for urging the conductor toward the paper so that contact is made between the conductor and the paper, a circuit for imposing a voltage between the conductor and the paper to cause a current to flow between the paper and the conductor with sufficient density to mark the paper, a phase shifting network, means for applying the signal to the network to get an output which is out of phase with the input, and means for modulating the voltage between the conductor and the paper with the network output.

10. An oscillograph comprising means for producing a magnetic field, an elongated movable conductor, means for supporting the conductor in the magnetic field with the conductor extending transverse to the direction of the magnetic field, means for supporting and moving current sensitive paper past the conductor in the direction of the longitudinal axis of the conductor, means for urging the conductor toward the paper so that contact is made beween the conductor and the paper, an electrode disposed to make a sliding contact with the paper, there being some contact resistance between the conductor and the paper and between the electrode and the paper, a circuit for imposing a voltage between the conductor and the electrode to cause a current to flow between the paper and the conductor with sufficient density to mark the paper, and a series resistor in the circuit at least three times as large as the said contact resistance.

11. Apparatus according to claim 10 in which the series resistor is adjustable.

12. An oscillograph comprising means for producing a magnetic field, an elongated movable conductor, means for supporting the conductor in the magnetic field with the conductor extending transverse to the direction of the magnetic field, means for supporting and moving current-sensitive paper past the conductor in the direction of the longitudinal axis of the conductor, means for urging the conductor toward the paper so that contact is made between the conductor and the paper, a circuit for imposing a writing voltage between the conductor and the paper to cause a current to flow between the paper and the conductor with sufficient density to mark the paper, means for sensing the contact resistance between the paper and the conductor, and means responsive to the contact resistance for maintaining a substntially constant current at the contact between the paper and the conductor.

13. An oscillograph comprising means for producing a magnetic field, an elongated movable conductor, means for supporting the conductor in the magnetic field with the conductor extending transverse to the direction of the magnetic field, means for supporting and moving current-sensitive paper past the conductor in the direction of the longitudinal axis of the conductor, means for urging the conductor toward a zero position and the paper so that contact is made between the conductor and the paper, a circuit for imposing a writing voltage between the conductor and the paper to cause a current to flow between the paper and the conductor with suificient density to mark the paper, means for continuously decreasing the writing voltage as the conductor moves away from the zero position, and means for continouusly increasing the writing voltage as the conductor moves to ward the zero position.

References Cited in the file of this patent UNITED STATES PATENTS 1,958,696 Digby May 15, 1934 2,434,647 Waterman June 25, 1948 2,644,738 Gardner July-7, 1953 2,647,033 Faus July 28, 1953 

